“With crime fiction inevitably, you get a lot of reported action,” as Nick says. “Sometimes you see crime fiction on stage and you are being told what has happened and you are then getting a big series of questions about whatever it is that has already happened – rather than dramatising the actual action.

“We needed to find a way of avoiding a series of arid interviews and endless questions. You can’t totally get out of that because of the nature of detective fiction, but you have got to find a way of making it stylish and actually getting the audience to follow Holmes’ journey.”

The company is considerably helped by the fact that the audience are on board to start with. Every generation gets its own Sherlock Holmes. He is a character with an enduring place in this country’s heart.

“You think of the show with Benedict Cumberbatch and Martin Freeman and also maybe Sherlock Gnomes and also the films with Robert Downey Jnr and you see that Sherlock Holmes has been there since the Jeremy Brett TV shows and even before them. He is absolutely part of the literary fabric of this country.”

But Nick is happy to break new ground – to an extent.

“You see The Hound of the Baskervilles done quite a lot, but not really The Sign of Four. It is quite nice to approach a different one. I think it is better than A Study in Scarlet in terms of the actual intrigue and also seeing Holmes doing his work, seeing his deduction and his detection and how he works it out.”

In the play, when Mary Morstan arrives at Baker Street to request help following the mysterious disappearance of her father, Sherlock Holmes and his companion Dr Watson are plunged into a murky world of deception and a complex plot involving murder, corruption and stolen jewels.

Nick’s starting point is that he simply loves all crime fiction: “And with this, you have got the complete archetype, Holmes and Watson, the double act, that fantastic partnership. You see it in Morse and Lewis. You even see it in Cagney and Lacey.

“And there is also something fascinating about watching Holmes solve the case, this towering intellect in action, his almost stark lack of emotion. He talks about how marrying would ruin his powers of deductive reasoning.”